1. Field of the Invention
This invention relates to multiplex transmission systems in vehicles, and more particularly to a multiplex transmission system in a vehicle which transmits data on the opening and closing of an ignition switch through a plurality of circuits.
2. Related Art
An example of a conventional multiplex transmission system in a vehicle is a power distributing system in a vehicle which has been disclosed, for instance, by Japanese Patent Application (OPI) No. 146080/1993 (the term "OPI" as used herein means an "unexamined publication application").
The power distributing system will be described with reference to FIG. 7. A vehicle 50 is divided into three parts: an engine room 51, a cowl 52, and a rear room 53. In the engine room 51, two parts, namely, a power source 54, and a distributing unit 56 are provided, the distributing unit 56 being connected through a power line 55 to the power source 54. In the cowl 52, a distributing unit 59 is provided which is connected through a power line 58 to the distributing unit 56 in the engine room, and in addition a meter 60 and a switch 61 are provided which are connected to the distributing unit 59. In the rear room 53, a distributing unit 62 is provided which is connected through a power line 57 to the distributing unit 56. A common multiplex transmission line 63 is connected to those distributing units 56, 59 and 62. The distributing units 56, 59 and 62 control the outputs of loads (not shown) which are connected thereto.
The system operates as follows: When turned on, the switch 61 provides an "on" signal. The "on" signal is applied to the distributing unit 59, where it is converted into a multiplex transmission signal. This signal is transmitted through the multiplex transmission line 63 to the other distributing units 56 and 62, so that those distributing units 56 and 62 detect the fact that the switch 61 has been turned on, and do necessary actions.
However, the above-described conventional system is disadvantageous in the following points: That is, when the distributing unit 59 is out of order, then the "on" signal of the switch 61 is not transmitted to the other distributing units 56 and 62. As a result, not only is the load connected to the distributing unit 59 disabled, but also the loads connected to the distributing units 56 and 62 are disabled. Hence, the functions of the vehicle are all lowered at the same time.